1. Field of the Technology
The disclosure relates to the field of microfabrication, specifically microfabrication of high quality three dimensional structures using wafer-level glassblowing of fused quartz and ultra low expansion glasses.
2. Description of the Prior Art
Conventional high performance gyroscopes and resonators are fabricated in macroscale using precision machining techniques. This results in large devices (approximately one inch diameter or more as opposed to 1 mm diameter), with large power consumption, and high cost. At the same time, conventional microelectromachined (MEMS) devices, while small and low power, are limited to two dimensional architectures and have poor performance.
Perhaps the most widely known form of vibratory rotation sensors employs three hemispherical shells as vibratory elements to detect rotation about three mutually orthogonal axes. Known in commercial avionics as hemispherical resonator gyros (HRG), these devices provide a very high degree of accuracy and sensitivity at low rotation rates as required by inertial grade navigation systems. Other features of HRG include longer useful operating life, higher reliability and a more cost effective system than many alternative rotation sensing technologies for commercial and military aviation.
Also called a hemispherical resonator gyroscope or HRG, a wineglass resonator makes use of a hemisphere driven to resonance, the nodal points of which are measured to detect rotation. There are two basic variants of the system, one based on a rate regime of operation and one based on an integrating regime of operation, usually in combination with a controlled parametric excitation. It is possible to use both regimes with the same hardware, which is a feature unique to this type of gyroscope. Maximization of the quality (Q) factor is key to enhancing performance of vibratory MEMS devices in demanding signal processing, timing and inertial applications. The macro-scale hemispherical resonator gyroscope (HRG) with Q-factors over 25E+6 motivates the investigation of 3-D fused quartz micro-wineglass structures for use as vibratory elements.
With the emergence of novel fabrication techniques, the batch fabrication of 3-D wineglass structures is becoming possible. For instance, hemispherical shells fabricated by deposition of polysilicon or silicon nitride thin films into isotropically etched cavities have recently been demonstrated. Alternative fabrication techniques include “3-D SOULE” process for fabrication of mushroom and concave shaped spherical structures as well as blow molding of bulk metallic glasses into pre-etched cavities. However, MEMS wineglass resonators with sufficient smoothness, low anchor losses and low thermoelastic dissipation (TED) have not yet been demonstrated in the literature. To take full advantage of the 3-D wineglass architecture, fabrication techniques with low surface roughness as well as materials with high isotropy and low thermoelastic dissipation are desired.
It has been demonstrated that MEMS devices can reach the fundamental QTED limit by using a combination of balanced mechanical design and vacuum packaging with getters. Thermoelastic dissipation is caused by local temperature fluctuations due to vibration and the associated irreversible heat flow, which results in entropic dissipation. Thermoelastic dissipation can be reduced either by decoupling the mechanical vibrations from the thermal fluctuations or by using materials with low coefficient of thermal expansion (CTE). This current illustrated embodiment of the invention focuses on materials with low CTE, such as fused quartz (0.5 ppm/° C.) or ultra low expansion titania silicate glass (0.03 ppm/° C.), which can provide a dramatic increase in fundamental QTED limit (QTED>7E+10 for a TSG wineglass). However, when compared to silicon, titania silicate glass and fused quartz dry etching suffers from order of magnitude higher surface roughness, lower mask selectivity and aspect ratios.
Pyrex glassblowing at 850° C. on a silicon substrate has been previously demonstrated for fabrication of smooth, symmetric 3-D structures. However, TSG glassblowing requires upwards of 1600° C. glassblowing temperature due to its higher softening point, which prevents the use of fabrication processes that rely on a silicon substrate. The current illustrated embodiment of the invention as detailed below explores the hypothesis that high temperature glassblowing (1650° C.), may serve as an enabling mechanism for wafer-scale fabrication of TSG/fused quartz 3-D wineglass structures.
What is needed therefore is an apparatus and method to bridge the gap between conventional macroscale gyroscopes and previous MEMS devices by enabling high volume and low cost manufacturing of ultra high quality three dimensional MEMS devices using advanced materials, which are not amenable to conventional MEMS fabrication.